This invention relates generally to a load responsive fluid power system using a servo valve in control of a fluid motor driving a load and operated by a variable displacement pump.
In more particular aspects this invention relates to a load responsive fluid power system in which the pump displacement control is regulated in response to a load pressure signal supplied from a fluid motor.
In still more particular aspects this invention relates to a variable displacement pump control which automatically regulates the outlet flow of the pump to maintain a constant preselectable ratio between the pump outlet pressure and servo valve controlled motor load pressure and to vary the pressure differential between pump outlet pressure and motor load pressure with variation in magnitude of the load.
In still more particular aspects this invention relates to a multiple load system in which the pump control responds to highest system load.
Load responsive fluid power and control systems are very desirable for a number of reasons. They permit load control with reduced power losses and therefore, increased system efficiency and when controlling one load at a time provide a feature of flow control, irrespective of variation in the magnitude of the load. Normally such valves transmit load control signals to the pump control, which automatically maintains pump discharge pressure at a level higher, by a constant pressure differential, than the pressure required to sustain the load. A variable orifice introduced between pump and load varies the flow supplied to the load, each orifice area corresponding to a different flow level, which is maintained constant irrespective of variation in magnitude of the load. Load responsive valves for such a system are disclosed in my U.S. Pat. No. 3,470,694 dated Oct. 7, 1969 and also in U.S. Pat. No. 3,455,210 issued to Allen on July 15, 1969. Load responsive variable displacement pump control for such a system is disclosed in my U.S. Pat. No. 3,444,689 dated May 20, 1969. The universal application of such a system is, however, limited by several basic system disadvantages.
Since this system, to maintain flow characteristics of system valves and to retain high system efficiency, works with a comparatively low constant pressure differential between the pump outlet pressure and the load pressure, the use of the conventional servo type valves in such a system becomes impossible. A servo valve, well known in the art, takes a low energy input and translates it into high energy hydraulic output. Feedback, well known in the art, keeps the hydraulic output of flow or pressure proportional to input signal. It is generally accepted that a servo valve can deliver maximum power to the load when the load differential pressure equals two thirds of the supply pressure. Therefore, neglecting other losses, one third of the system pressure drop occurs across the servo valve. This optimum pressure drop can be influenced by matching servo valve size to the load, but increasing servo valve size increases weight and cost of the system. If the servo valve is too small, the system may become velocity limited during its duty cycle and have poor dynamic response. When the system operates under this condition, a large portion of the pressure drop occurs across the servo valve instead of across the actuator connected to the load, and little useful work is done. The power expended in the servo valve heats the working fluid excessively which is undersirable and inefficient. Maximum system pressure and flow used by the servo valve, in control of a load, is dictated by the most adverse combination of load requirements, which must be satisfied, but which in the average system occur only during a very small percentage of the time and usually do not occur in the normal duty cycle. Due to their basic characteristics servo valves, almost without exception, are used in constant pressure systems, which are supplied by variable displacement pumps controlled by conventional pressure compensator controls, which automatically vary the displacement of the pump to maintain a constant system pressure. The constant pressure level of the system is determined by the size of the servo valve and most adverse combination of load requirements. In consequence the servo valve uses constant maximum pressure in control of a variable load, most of the fluid power energy being converted by throttling into heat, which not only represents system inefficiency, but introduces additional parasitic effects, well known to those skilled in the art. High pressure drops across servo valve when controlling small loads with high system pressure produce many other adverse effects, like change in the gain of the system, high flow forces acting on the valve spools and possible system instability.